Oil and natural gas are often found together in the same reservoir. The composition of the raw natural gas extracted from producing wells depends on the type, depth, and location of the underground deposit and the geology of the area. During production, oil, gas, and water flow to the surface, passing as an emulsion or a mixture.
During a well's flowing life, as rates and gas velocities decrease, liquids will start to collect at a well bottom, causing a gradual increase in back pressure. Fluid buildup may cause the lifting efficiency of a well to decrease and in some cases, may even cause a well to cease to flow.
Operators may use any number of artificial lift techniques to raise fluid to the surface after a well slows or ceases to flow. One known method comprises plunger lift. The function of the plunger is to prevent fluid buildup from accumulating to the point that they would cause a decrease in rate or cause the well to no longer flow.
The operation of a plunger lift system relies on the natural buildup of pressure in a well during the time that the well is shut in at the surface by a wellhead controller (or in an “off” mode). When a well is shut in, pressure is allowed to build up. In a shut in mode, no production occurs at the surface. When pressure has sufficiently built up to enable the accumulated liquids in the tubing to be lifted along with the plunger, the well is opened to production. A plunger lift system operates to “lift” oil or water and natural gas from a well bottom during natural gas production when the well is in an “on” mode, thus unloading fluid buildup and increasing the productivity of oil and natural gas wells. Functionally, the plunger provides a mechanical interface between the produced liquids and the gas. This mechanical interface minimizes liquid fallback which thereby boosts a well's lifting efficiency.
In the industry, the optimization of plunger lift has primarily focused on changing the on/off cycle time based on factors such as time, differential pressure, plunger arrival speeds, etc. In fact, most plunger lift controllers commonly pre-set a minimum off time or fall time on the premise that this minimum time will allow the plunger to fall safely to the bottom of the well before the on time cycle is enabled. Since minimizing the shut-in time is an important part of the optimization process, it is important to know when the plunger has reached the bottom of the well.
It is well-known in the industry that the science of determining fall time can be imprecise without the use of very specialized equipment. In general, operators often determine that the plunger is on bottom based on an arbitrary interval of time, a guess, or an estimate based on the type of plunger, its estimated fall velocity and the depth of the well. For example, an operator can assume it takes a plunger 45 minutes to travel to well bottom. This travel time is typically referred to as “fall time,” which can be the actual or estimated interval of time when a motor valve is shut to close the flow line and when the plunger hits bottom. Many factors, however, can affect the actual fall time of a plunger. Different types and brands of plungers fall at different rates. For example, a 2⅜″ pad-type plunger can have a fall time of about 48 minutes depending on the depth of the well. In the same well, a bar-stock plunger might fall in about 22 minutes; a by-pass plunger could reach bottom in as little as seven minutes. In addition, new plungers have been observed to fall at different rates than worn plungers.
Fall time can also be a function of a well's depth and the amount and composition of liquid in the well. Well maturity can also alter plunger fall times. As a well matures, it can produce more or less fluid through which a plunger falls. In addition, the presence of salt, sand, or solids can have an influence on how quickly the plunger reaches bottom. Well bore features can also affect fall time. Such features can include but are not limited to the condition of the tubing, whether the tubing is rough or smooth, the type of rod-cuts, the existence of tight spots, scale, and/or paraffin build up and the well's trajectory (vertical vs. deviated). Other conditions affecting plunger fall time would be known to those skilled in the art.
Therefore, a need exists for a method for determining when a plunger reaches the bottom of an oil or gas well equipped with a plunger lift system.